A device for respiratory assistance is already known from the patent EP-A-0390684 and comprises:                a tube which forms a main channel and which is designed to be connected by its distal end to a respiratory airway of a patient such that said main channel connects the respiratory system of said patient to the outside; and        at least one auxiliary channel, made for example in the wall of said tube and allowing injection of a stream of breathable gas intended to ventilate the patient, this auxiliary channel emerging into the main channel in front of the distal end of the latter.        
In such a device, the breathable gas feeding the auxiliary channel is in most cases pure oxygen. However, some patients whose bodies are accustomed to a high level of carbon dioxide in the blood cannot tolerate ventilation with pure oxygen, which would lead to a risk of heart problems.
Therefore, in order to overcome this drawback, it has already been proposed, in the document EP-A-1441791, to use a respiratory assistance device of this kind that additionally comprises, between the distal orifice of the auxiliary channel and the distal end of the main channel, controllable means of fluid communication which, in the open position, are able to form a passage of variable cross section connecting said main channel to the external environment, in such a way that external air is aspirated through the means of communication by the stream of breathable gas of the main channel. The air thus introduced dilutes the breathable gas, which can then be tolerated by those patients referred to above.
The aspiration of external air arises from the vacuum generated by the stream of breathable gas in the main channel, downstream of the distal orifice of the auxiliary channel. However, the vacuum created proves not only to be low but also very unstable. This is because it is subject to variations that are unpredictable and difficult to control and that cause irregular aspiration of outside air.
Moreover, the person operating the respiratory assistance device on the patient may be forced to continually adjust the variable cross section of the passage formed by the means of communication (and thus the strength of the aspiration) in order to ensure that the dilution of the respiratory gas by the air remains substantially constant in the main channel.